1. Field of the Invention
This invention relates to biofouling reduction, and, more particularly, to the reduction and prevention of biofouling in facilities utilizing water, such as sea water, carrying biological organisms.
2. State of the Art
Control of fouling in facilities utilizing process water has been a long-standing problem. Macroorganisms, such as species of mussels, including zebra mussels, found in sea water and fresh water sources, such as the Great Lakes, have become notorious sources of biofouling. With their ability to grow profusely and in great concentration, mussels, including zebra mussels, have been known to completely block and close large diameter water inlet pipes for the cooling systems of major seaside and lakeside power plants. While less notorious, microorganism fouling can be just as troubling. Such fouling can reduce heat transfer through the piping and reduce the flow of water through the piping of heat exchangers, thereby decreasing their ability to discharge heat into the water. In addition, such fouling can adversely change the permeability of filters (and, more particularly, the permeability of the filtration media contained within the filters). In addition, macroorganisms and microorganisms within piping systems generate chemical waste products that induce and promote chemical corrosion within the systems. This phenomenon, commonly known as microbially induced corrosion, attacks the structural integrity of piping systems.
Various techniques have been proposed to prevent, or at least reduce biofouling, but all have had their limitations. Among the early attempts was the use of heavy dosages of chlorine ions as a biocide to kill the organisms. While this approach gave the desirable result of sanitizing the water, it also produced undesirable excess hypochlorous acid which itself attacked the structural integrity of the piping system. More recently, environmental concerns have been raised about the high doses of chlorine and, in particular, the discharge of residual (or unreacted) chlorine and reaction products, such as trihalomethanes, from the piping system back into the ecology.
Similarly, high doses of copper ions have been proposed as a biocide. As with chlorine, the discharge of high levels of residual copper ions back into the ecosystem presents a significant environmental concern. In addition, it has been found that at least certain microorganisms have responded to copper ion treatment by developing a degree of resistance to this biocide. Copper ion alone is regarded as effective only against macrofouling. Moreover, typically, electrolytic cells utilizing copper electrodes are used to generate the copper ions, and these electrodes experience a high rate of sacrificial loss in generating the needed dosage level of ions.
A more recent and more promising approach was jointly developed by certain of the inventors of the instant invention in their U.S. Pat. No. 4,869,019, incorporated herewith by reference. This patent describes the synergistic effect of low dosage levels of chlorine ions used in conjunction with low dosage levels of copper ions to form a treatment additive sufficient to temporarily stress or disorient (but not "necessarily" kill) both macroorganisms and microorganisms so that they pass through the piping system of a facility without attaching themselves to the system. Being of low dosage, the chlorine and copper ions generated by this technique represent significantly less environmental concern than the previous techniques. As successful as this combined ion treatment approach may have been, it still suffered limitations as applied to large facilities. Combined ion treatment is effective for only a relatively short time duration (such as, for example, thirty (30) minutes). Thus, if combined ion treatment is used only at the water inlet to the piping system of a large facility, the treatment is effective for only part of the travel of the organisms through the piping system. Fouling can then occur in the downstream part of the piping system for which treatment effectiveness has been lost. Conversely, use of combined ion treatment at numerous points along the piping system requires a corresponding number of sources of ion generation, such as electrolytic cells for generating the treatment ions, with resultant increased capital costs and operating expenses, and can result in an environmentally unacceptable buildup of certain of the ions at discharge.
Among the facilities not adequately treated for biofouling by these prior art techniques are desalinization plants. These plants use reverse osmosis semipermeable membranes to remove inorganic ions, such as salt, from sea water or fresh water brines. However, biological organisms carried along with the sea water and brines tend to grow on the semipermeable membranes of these cells, causing them to lose permeability and thus to lose salt removal efficiency. This lost efficiency at times has exceeded 50%, thereby reducing fresh water production or requiring additional production capacity. Typically, biofouling treatment of such facilities takes the form of adding high dosage levels of chlorine ions at the inlet to the piping system. While this may sanitize the water of organisms, the high dosage level of chlorine itself can, in some instances, chemically react with the media and adversely affect its permeability. Moreover, the generation of high dosage levels of chlorine is expensive in terms of capital equipment required and operating expenses, and the disposal of such levels of chlorine can present environmental problems.
Another difficult biofouling problem is presented by marine fire water systems. These systems are found on board ships, oilfield offshore rigs and production and storage facilities, and take the form of a ring main with fire extinguishing sprinkler and deluge system utilizing sea water constantly charged under pressure in the system. Over time, the biological organisms in the water grow, stimulating the production of corrosion product and blocking the piping system, thereby preventing water discharge when needed. Prior art systems called for a constant, relatively small volume discharge of sea water from the system and the delivery of high dosage levels of chlorine at the water inlet to the system. However, chlorine at these levels causes and enhances corrosion of the piping system and presents environmental problems at discharge. Other approaches for solving this problem include the use of high alloy brass, such as Admiralty Brass, as the material of construction of the piping system. Such materials leach copper ions to retard biological growth, but are expensive and difficult to install.
Further biofouling problems arise with facilities having numerous points requiring biofouling treatment, such as, for example, power plants having a bank of heat exchangers in parallel flow arrangement, and oilfield water injection apparatus for injecting treated water into a water bearing formation of a hydrocarbon reservoir having a number of filter units in series or parallel flow arrangement. The typical prior art treatment technique for such multiple treatment point facilities is to deliver a large dose of chlorine to the piping inlet to the facility, thereby also delivering chlorine at the same time at, or above, the desired effective dosage levels to all of the numerous treatment points downstream of the piping inlet. Because of the loss of treatment effectiveness of chlorine over time, the dosage level of the chlorine at the inlet in single point treatment systems must be high enough that enough chlorine remains to be effective at the treatment point farthest removed from the inlet. However, this approach requires that large quantities of chlorine be provided at the inlet with resultant high capital cost and operating cost and exposes the piping system to high levels of highly reactive chlorine ions.